Heat a sous vide water bath to 125F degrees. See post above for additional cooks times depending on doneness preferences. This will give you a rare steak.
Season the steaks with salt and pepper and place them in a vacuum seal bag.
Cook for 1 ½ to 2 hours.
Remove from bag and pat dry with paper towels.
Heat a skillet over high heat until smoking and add the avocado oil.
Place the steaks in and cook for 45-60 seconds per side.
After flipping the steaks over, add the butter to the skillet and baste the steaks for the remaining 45-60 seconds (spooning butter over top of the steak).
Remove, slice against the grain and serve.
Notes
Make sure the bag does not float while cooking. It should be fully submerged. If it floats, add sous vide weights/magnets or a heavy spoon, towel or a sous vide rack to keep it submerged.
Pat the steak fully dry with paper towels before searing. The drier the steak is, the quicker it will form a crust when searing.
If not serving right away, place the steaks in an ice bath for 15-20 minutes when you remove the bags from the sous vide water bath (do not open the bag - keep the steaks sealed in the bag). Store in the fridge for up to 5 days, or in the freezer for up to 3 months. Thaw in the fridge overnight and sear when ready to serve.
You can also use a grill or a kitchen blow torch instead of a cast iron skillet.